When using Magic Lantern for the 550d/T2i, an SD card with EOS_DEVELOP and BOOTDISK flags is needed to mark the card for autobooting. However, due to a limitation in the CardTricks software, it is not possible to write the flags to the MBR of SD cards larger that 4 gigabytes in size. To add these flags from a Windows machine easily, download bootCF from here,and cc3280mt.dll from hereor any other trustworthy .dll download site. For Mac users, the terminal commands listed in the original 550d/T2i hack announcement should work regardless of card size.